The present invention relates to a device for and a method of sensing motor current in an electric power tool.
Most electric power tools incorporate an electric motor as the source of mechanical power. The motor current will surge exponentially when the motor is stalled as a result of mechanical obstruction, which can be hazardous and therefore over-current protection is often employed.
In a typical arrangement, the over-current protection is implemented by a tripping circuit incorporating a current transformer including a toroid and a winding. The toroid extends around the live cable of the motor to detect or sense the motor current for output at the winding. Whenever the motor current rises to a dangerous level, the tripping circuit will come into action and switch off the electrical power supplied to the motor.
The current transformer tends to be bulky in size and/or expensive if it is required to handle a relatively large motor current, for example, up to or beyond 30A.
The invention seeks to mitigate or at least alleviate such a problem by providing an improved device for and method of sensing motor current in an electric power tool.
According to a first aspect of the invention, in an electric power tool comprising an electric motor supplied with electrical power via a live cable and incorporating over-current protection implemented by a tripping circuit including a current sensor associated with the live cable, there is provided a device for sensing the motor current for over-current protection. The device comprises a part of the sensor and at least one cable segment having opposite ends connected in parallel to a section of the live cable across two junctions such that the motor current will be distributed between the cable segment and the live cable section in predetermined proportions. The sensor part extends around one of the cable segment and the live cable section for sensing the respective proportion of the motor current.
Preferably, the cable segment has substantially the same electrical resistance per unit length as said live cable section.
It is preferred that the cable segment have substantially the same length as the live cable section.
It is further preferred that the cable segment have substantially the same electrical resistance per unit length as the live cable section. Preferably, the sensor part extends around the cable segment.
In a preferred embodiment, the motor current sensing device comprises two cable segments, each of which has substantially the same electrical resistance as the live cable section.
According to a second aspect of the invention, in an electric power tool comprising an electric motor supplied with electrical power via a live cable and incorporating over-current protection implemented by a tripping circuit including a current sensor associated with the live cable, there is provided a method of sensing the motor current for over-current protection, The method comprises providing at least one cable segment having opposite ends, and connecting the cable segment at opposite ends in parallel to a section of the live cable across two junctions such that the motor current will be distributed between the cable segment and the live cable section in predetermined proportions. The method includes arranging a part of the sensor to extend around one of the cable segment and the live cable section for sensing the respective proportion of the motor current.
It is preferred that the junctions are made by way of 15 spot welding.
Preferably, providing at least one cable segment includes choosing the cable segment to have substantially the same electrical resistance per unit length as the live cable section.
It is preferred that providing at least one cable segment includes choosing the cable segment to have substantially the same length as the live cable section.
It is further preferred that providing at least one cable segment includes choosing the cable segment to have substantially the same electrical resistance per unit length as the live cable section.
Preferably, the step of arranging the sensor part is to arrange the sensor part to extend around the cable segment.
In a preferred embodiment, providing at least one cable segment comprises providing two said cable segments having substantially the same electrical resistance as the live cable section.
It is preferred that the step of connecting the cable segment comprises making the junctions by way of spot welding.